Comparative biological approaches will provide novel insights into aging. We propose a "functional comparative' approach to the biology of aging in a range of animal models to test the hypothesis that "aging is strongly influenced by an ancient and conserved genetic pathway that coordinately regulates cell cycle progression, cell death and stress response." The study is prompted by our unexpected observation that the lifespan and stress resistance of Caenorhabditis elegans is under the influence of cell cycle checkpoint components. This taken together with recent speculation on the role of the tumor suppression and apoptosis p53 protein in aging, we believe it is timely to undertake a inter-specific comparative study of p53 and checkpoint functions. As these factors appear to affect aging in both simple invertebrates and mammals, we expect to define broad mechanisms by which cellular and organismal aging are co-regulated.